The present invention relates to the field of post-tension tendons for constructing concrete structures. More particularly, the invention relates to a corrosion resistant, unbonded mono-strand tendon system for post-tension construction.
Mono-strand tendons for unbonded post-tension construction typically comprise a seven wire strand tendon placed within an elastomeric sheath. The seven wire tendon is formed with six wires helically wrapped around a central core wire. Grease or another lubricant is placed on the outer surface of the seven strand wire tendon adjacent to the elastomeric sheath to facilitate movement between the tendon and the sheath, and to resist corrosion created by air and water infiltration between the tendon and sheath.
Tendon corrosion is a significant concern in post-tensioned systems. Such corrosion occurs when water, salt and other corrosive agents contact the tendon materials. Because the strength of post-tension concrete systems depends on the tensile strength of the steel tendons, failure of the tendons can lead to failure of the entire structure. Tendon failure typically occurs due to water intrusion into the interstices between the tendon and surrounding concrete. Certain environments around salt water and other highly corrosive factors require extra caution in designing special corrosion resistant post-tension systems.
The installation of post-tension tendons typically occurs in a rugged construction environment where the tendons can be damaged by equipment, careless handling, and contact with various site hazards. When the elastomeric sheath is punctured, a water leak path into contact with the tendon is established. The puncture must be patched to resist water intrusion between the sheath and tendon as concrete is poured around the post-tension tendon, and before the concrete cures. The puncture and patch can create a discontinuity between the tendon and the sheath, and this discontinuity can impede proper post-tensioning of the tendon after the concrete has cured.
One conventional technique for providing extra protection in corrosive environments is to increase the thickness of the plastic sheath covering the tendon. A plastic sheath at least forty mils thick is formed around the tendon to resist abrasion and puncture damage. Although this approach provides incremental protection against leakage, a thicker sheath does not provide redundant protection to the tendon steel.
Another anti-corrosion technique for providing corrosion resistance uses tendon end sealing systems having seals and grease-filled pockets for blocking water intrusion and to resist water intrusion into the tendon core. Intermediate cover caps permit passage of the sheathed tendon during installation, and grease-filled end cover caps seal the tendon end against water intrusion. Oil or grease is sometimes pumped into the end of the tendon end to fill the interstices at the tendon ends, however this procedure does not protect the internal wire strands forming the tendon. The penetration depth of end seal protection is sometimes extended by short corrosion protective sleeves or adapter tubes which extend for several feet from the end cap into the concrete. Such adapter tubes have a seal around the tendon exterior surface and form have a pocket for packing grease or other corrosion inhibitor near the tendon ends.
Another technique for resisting high corrosion environments is to specially treat the individual wire strands within a mono-strand tendon. One such process coats each wire strand with an electrostatic fusion-bonded epoxy to a thickness between one and five mils thick. Similar wire strand techniques use galvanized wire and other corrosive resistant wires within the multiple wire tendons to form a corrosion resistant tendon.
Another conventional post-tension system for highly corrosive environments uses a seamless plastic tube secured to encapsulated anchors at each end. The mono-strand tendon is placed within the plastic tube and is theoretically protected from water intrusion within the cavity formed by the plastic tube. However, a puncture or leak at any point along the plastic tube or at the connections between the tube and the end anchors can permit water intrusion into contact with the mono-strand tendon, thereby permitting corrosion to occur.
Significant effort has been made to create improved corrosion resistant materials compatible with the exterior sheaths and resistant to corrosion. Corrosion resistant materials typically have an affinity to metal and are capable of displacing air and water. Additionally, such materials are relatively free from tendon attacking contaminants such as chlorides, sulfides and nitrates. However, the effectiveness of such corrosion resistant materials is limited by the system design placing such materials into effective contact with the individual tendon wire strands.
A need exists for improved post-tension tendons which resist corrosion and consequential failure of the post-tension structure. The tendons should be compatible with existing tensioning procedures and should resist the risk of water intrusion into contact with the internal wire strands.
The present invention discloses an unbonded post-tension tendon comprising a mono-strand tendon formed with at least two wire strands and having an exterior surface and having interior interstices between said wires, a first sheath around the tendon exterior surface, and a corrosion resistant material positioned within the tendon interstices and between the tendon exterior surface and the first sheath.
In various embodiments of the invention, a second sheath can be positioned about an exterior surface of the first sheath and a lubricant or a corrosion resistant material can be positioned between the first and second sheaths. The mono-strand tendon can comprise six wires helically wrapped about a center wire to form helical grooves on the tendon exterior surface, and the first sheath can have a thickness less than ten millimeters tightly wrapped about the tendon to form helical grooves in the first sheath exterior surface. Corrosion resistant material can be placed in the helical grooves between the tendon exterior surface and the first sheath, or in the helical grooves between the sheath exterior surface and the second sheath.